The present invention relates to a process for forming a phosphate conversion on iron or steel. More particularly, it relates to a composition and process for forming a nickel phosphate coating on iron or steel. The applied coating is suitable as an undercoating for painting, as a solid lubricant for cold working or as an undercoating for ceramic or porcelain enameled ironwork.
In order to improve corrosion resistance and adhesive properties, surfaces of iron or steel have previously been coated with a film of a slightly soluble metal phosphate. For this purpose, the surface has been reacted with an acidic aqueous metal phosphate solution which resulted in the inclusion of the metal cation in thus-formed film. Originally, iron phosphate or manganese phosphate was used. In recent years, however, zinc phosphate compounds have been employed. Besides the zinc ion, these phosphate conversion solutions often contain added film forming divalent metal ions such as calcium, manganese, iron, nickel, cobalt or cadmium ions.
In such cases, these added cations represent a minor proportion of the film-forming cations with the zinc ion predominating.
Moreover, it is well known that such phosphate conversion solutions may be modified with various additives such as Cu salts, Ti salts, boric acid, silicic acid, as well as single and complex fluorides. Zinc phosphate conversion solutions to which a minor amount of nickel ion is added have been hitherto used as a phosphate conversion solution, but this added nickel ion is effective only as a reaction accelerator to produce the zinc phosphate film. The amount of the crystalline nickel phosphate existing in the zinc phosphate film is so extremely small that X-ray diffraction cannot detect it. Japanese Prepublished Application No. 14028/72 discloses a process of phosphate conversion coating using a nickel phosphate bath, but in this case, the formation of crystalline nickel phosphate coating film was difficult and the resulting film was so very thin that it was difficult to detect the nickel phosphate by X-ray diffraction.
There are known processes designed to precondition the surface by immersing the surface in a titanium phosphate solution or by spraying the same solution. It is also known to precondition the surface by immersing the material in an aqueous suspension of fine powders of zinc phosphate, iron phosphate or calcium phosphate or by spraying the same suspension on the material. Following preconditioning, the surface is then treated to form the adherent zinc phosphate coating by treating the preconditioned surface with zinc phosphate conversion solution. It was found difficult, however, to form a crystalline nickel phosphate coating on such preconditioned surfaces by treatment with a nickel phosphate conversion solution.
These conventional phosphate conversion processes have not been entirely satisfactory. For example, in the case of phosphate conversion of strip material, it is difficult to obtain an adherent phosphate film in a short time. In addition, the phosphate film obtained by the conventional method lacks the desired stability in mechanical treatment such as bending and deep drawing, and in such cases cracks or peeling of phosphate film or of paints thereon are observed.